HUMANS usually have 46 chromosomes, but 22 of those are copies.
Each cell of the body contains a full set of chromosomes, and humans could have as many as 37 trillion cells each, so that’s 37 trillion copies of our 46 chromosomes in each human.
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What are chromosomes?
Chromosomes are thread-like structures of DNA, which is genetic material.
Every animal and plant cell has chromosomes.
Chromosomes come in pairs, wound together in a spiral and spooled up on a kind of protein spindle inside the nucleus of the cells of animals and plants.
A full set of chromosomes contains the DNA necessary to create the whole animal or plant.
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Cells like egg and sperm contain half the genetic information and that is what is passed on to the offspring, children.
The DNA on a chromosome is like a thread of genes interspersed with other DNA, that does other jobs such as regulating genes, or its ancestral genetic material.
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How many chromosomes do humans have?
Humans have 46 chromosomes in 23 pairs.
We inherit 23 chromosomes from our mother and 23 chromosomes from our father.
The chromosomes in the first 22 pairs are identical in a normal cell and they are the same in both genders.
The 23rd pair is the sex chromosome and therefore determines the sex of the individual.
This chromosome is either XX for female or XY for male.
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How many genes do humans have?
There are estimated to be between 20,000 and 25,000 genes that code for human proteins, but this number has changed a lot over the last few decades.
Around the year 2000, when gene sequencing was still quite a new idea, scientists estimated they might find as many as 100,000 human genes.
Part of the problem with counting the number of human genes is that scientists still don’t agree on exactly what a gene is.
Some geneticists think a gene is a sequence of DNA that codes for a human protein.
Others consider DNA sequences that regulate the function or expression of others as genes.
However it is defined, there can be more than one gene that codes for a particular characteristic.
Also, each gene in the body may hold the code for as many as 10 different proteins.
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What happens if you don’t have 23 pairs of chromosomes?
Having an unusual number of chromosomes can cause conditions such as Down’s syndrome, but the effects depend on which chromosomes are involved.
An organism grows by each cell making an exact copy of itself.
The chromosomes play a vital role in this copying process.
A similar process is used to make cells carrying half our DNA when we are creating children.
Sometimes mistakes are made in the copying process, and we end up with one or three of a chromosome instead of a pair.
Human chromosomes are usually numbered in their pairs: 1 to 22, and pair 23 are the sex chromosomes, where females have x and x, and males have x and y.
Sometimes a chromosome pair can lose a chromosome in copying (called monosomy), or gain a chromosome (trisomy).
An unusual number of chromosomes is rare, and it is more common to have an extra chromosome than one less.
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Very often an embryo with an unusual number of chromosomes won’t survive long, and probably not long enough to have been noticed.
The chromosomes most often seen in different numbers are:
- Three chromosomes 21, which is called Down’s syndrome
- Three copies of chromosome 18
- Three copies of chromosome 13
- Only one copy of chromosome x: Turner syndrome;
- Two copies of chromosome x and one copy of chromosome y: Klinefelter syndrome;
- One copy of chromosome x and two copies of chromosome y
- Three copies of chromosome x
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